Figure 2. Composite stratigraphic section of the Cambrian rocks of the Parahio Formation in the Parahio Valley, Spiti region, with microfossil occurrence and local ranges, and trilobite biozonation. Unzoned intervals refer to relatively thick intervals of section that yielded no trilobites. (4) and (5) Informal stages of the Cambrian System. The boundary between them is represented at the first occurrence of O. indicus. sh, shale; st, siltstone; vfs and fs, very fine sandstone and fine sandstone, respectively; carb, carbonate.